I. Field of the Invention
The present invention relates generally to multi-access communication systems. More specifically, the present invention relates to a control interface protocol for telephone sets for use in a satellite telephone system.
II. Related Art
Many communication systems, such as satellite systems, multidrop telephone lines, and multitap bus systems use multi-access communication configurations. Generally, in a multi-access communication system, a plurality of nodes are connected to a single multi-access channel or an interface bus. Each node has a queue of packets, representing data to be transmitted over or by the interface bus. The interface bus views all waiting packets as one combined queue to be served by an appropriate protocol.
Several methods for access to an interface bus have been developed for multi-access communication systems. One method is the xe2x80x9cfree-for-allxe2x80x9d method in which nodes normally send new packets immediately, hoping for no interference from other nodes. The problem, however, is that packets need to be retransmitted when collisions, that is, interference, occur. The other method is the xe2x80x9cperfectly scheduledxe2x80x9d method in which there is some order in which nodes are allocated reserved time intervals for interface bus access.
Multi-access channels are being considered for interfacing the components of various satellite telephone systems currently under development. For example, multi-access channels are being considered for providing service for fixed user terminal (UT) systems planned for use in some satellite communication systems. A fixed UT, also called as a subscriber unit or communications terminal, can be a telephone, a wireless unit or a fixed station. The fixed UT is designed to operate from a fixed location. Generally, the fixed UT includes a radio antenna unit (RAU) and one or more telephone sets (also called desksets) connected to the RAU.
A remote location with party line service can be efficiently linked to a satellite or other wireless communications system through a radio antenna unit (RAU). An RAU is a transceiver, comprising well known elements, that transmits and receives a modulated carrier signal to and from the satellite communications system through an antenna. The RAU generally includes a receiver, a transmitter and other signal processing circuitry required for wireless operations. The RAU provides receive and transmit capabilities for one or more telephone sets. During transmission, the RAU accepts audio signals from the multiple phones. An audio coder-decoder (or audio codec) in the RAU digitizes the audio signals, which are then used to modulate the carrier signal that is radiated to a satellite (or other relay apparatus) by the antenna. During reception, the RAU receives an input signal comprising a modulated carrier signal from a satellite. The RAU demodulates the input signal to retrieve the digital audio signal. After which, the audio codec converts the digital audio signal to an analog audio signal, and causes the analog audio signal to be sent to the multiple phones. The RAU can use a variety of additional signal processing and control elements as desired, and known in the art.
There are various advantages associated with the fixed UT. The fixed UT is relatively inexpensive to implement. The fixed UT appears to be especially suitable at remote locations, such as oil rigs, isolated rural areas, and the like where it may not be economically feasible to link such remote areas by land lines.
The fixed UT also appears to be suitable in areas where a typical wireless local loop may not be economically feasible. In a wireless local loop, multiple cellular telephones communicate via a base station (cell site or station). The base station is connected to a public switched telephone network (PSTN) through land lines. However, in some areas, there may not be enough wireless telephone users to justify the installation of a base station. There may also be a lack of sufficient public telephone network infrastructure. For those areas, a fixed UT with a single RAU being serviced by a satellite system or fixed relay stations is a preferred alternative.
The practical implementation of the fixed UT, however, still presents numerous challenges. What is needed is a suitable interface bus (that is, the multi-access channel) by which the RAU can be connected to multiple desksets. There is also a need for a method that will enable the desksets to share the interface bus and to prevent a collision in the interface bus when a plurality of desksets transmit packets concurrently. There is also a need for a control interface protocol that will enable the desksets to efficiently communicate with the RAU.
The present invention is directed to a control interface protocol for a wireless communication system such as a satellite telephone system. The satellite telephone system comprises a radio antenna unit (RAU) and a plurality of desksets connected to the RAU through an interface bus. A control interface protocol governs communications between the desksets and the RAU over the interface bus.
In one aspect, the present invention relates to a packet transmission system. According to the present invention, the RAU and the desksets communicate with each other using packets. Commands and data are sent in packets from the RAU to one or more desksets or from a deskset to the RAU. In one embodiment, each packet includes a start of header (SOH) byte, an address byte (ADDR), a command (CMD) byte, an argument (ARG) and a block check character (BCC). A packet begins with the SOH byte. The ADDR byte comprises a source and a destination address of the packet. The destination address allows an intended recipient to correctly receive the packet, while the source address allows a recipient to ascertain the originating source of the packet. The ADDR byte also includes a sequence number (SQN), as described in detail below.
According to one embodiment, the ARG is one byte when the most significant bit (MSB) of the CMD byte is 1, and the ARG has a variable length when the MSB of the CMD byte is 0. The BCC character is a parity check of all characters from SOH to ARG. The BCC character is formed by bit-wise exclusive OR (XOR) of each character sent, excluding those forming the SOH byte. All packets from the desksets, except negative acknowledgment (NAK) packets, are acknowledged by the RAU.
In one embodiment, the packets originating from the desksets include a source address between 1 and n, where n is an integer number that is equal to the total number of desksets connected to the RAU. A packet to a specific deskset includes a destination address, that is, the address of the deskset. A packet originating from the RAU to all desksets includes a first default address (for example, zero). A packet originating from the RAU to a deskset that does not have an address includes a second default address (for example, an integer number).
According to one embodiment, the packets are protected by horizontal and vertical parity checks. The BCC character is used for the vertical parity check. The bit at the end of each packet is used for the horizontal parity check.
In another aspect, the present invention relates to a method for handling error control for packets sent to the desksets (also referred to as xe2x80x9cterminalsxe2x80x9d) by the RAU (also referred to as the xe2x80x9ccommon nodexe2x80x9d). Each packet having modulo-sequential sequence numbers. The method includes the steps of sending a packet from the common node to one of the terminals; and sending a negative acknowledgment (NAK) from that one terminal to the common node when an error or unexpected sequence number is detected in the packet. That NAK includes the sequence number of the last valid packet received. Any lost packets are re-sent from the common node to that terminal when an unexpected sequence number is detected and a reboot command is sent from the common node to that terminal when the number of missed packets exceeds a predetermined threshold or when a NAK is received at the common node from the terminal.